Endogenous type C viruses have been isolated from several different baboon species by cocultivation of various tissues with heterologous mammalian cells. With H3-DNA transcripts prepared from these viruses, partially homologous nucleic acid sequences have been detected in the cellular DNA of all Old World monkeys and apes, including man. A comparison of the rates of evolution of viral gene sequences distinguishes those primates that have evolved in Africa from those that have evolved in Asia. Among the apes, only gorilla and chimpanzee are classified by these criteria as African, while gibbon, orangutan, and man are identified as Asian, leading to the conclusion that most of man's evolution has occurred outside Africa. We have also documented that several of the presently-known mammalian type C viruses arose by trans-species infection from distantly related species several million years ago. There are now four examples of this phenomenon. Type C viruses thus have a propensity for trans-species infection and subsequent integration into the germ cells of evolutionarily distant species.